There are many methods of creating an Arduino-powered lock, such as using a numeric keypad or a sequence of knocks as the "key" - however an interesting and simple method has been documented by Instructables member thallmls. This system is a combination lock without any visible buttons or keys - instead reed switches are mounted in random positions underneath the surface of the item to be locked.

These can be treated as normally-open buttons, and are activated when a magnet is placed above the top surface. Therefore by programming a sequence of "presses" - that is, passing the magnet over the reed switches in a particular order - the lock can be opened.

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